Mirrors are often classified into two categories, focusing and non-focusing. Non-focusing mirrors are typically flat and do not further enlarge or reduce an image being reflected by the mirror. Comparatively, focusing mirrors are generally curved and often enlarge or reduce an image being reflected by the mirror. Depending upon the curvature of the focusing mirror, the position of the object, and the position of the observer of the object, the image viewed by the observer may be enlarged or reduced, may be inverted or upright, and may be real or virtual.
Among the many applications available for focusing mirrors is their use in enlarging the field of view of an observer looking into the mirror. FIGS. 1 and 2 illustrate the basic optics associated with this enlarged field of view. In FIG. 1 the field of view (β) 110 available to the eye 120 of an observer looking into the flat mirror 100 from a distance d. As can be seen, as with all mirrors, the angle of incidence (α) 130 from the rays of an image behind the observer is equal to the angle of reflection (α) 140 of the rays of light being reflected by the mirror 100 to the eye 120 of the observer. The enlarged field of view available to an observer's eye looking into a focusing mirror is demonstrated in FIG. 2, which substitutes a curved convex mirror 200 for the flat mirror 100 of FIG. 1. As can be seen, when comparing FIG. 2 with FIG. 1, the field of view (β) 210 for the observer, as well as the angles (α) of incidence 230 and reflection 240, are greater for the same distance d, when the physically curved mirror 200 is substituted for the flat mirror 100.